This invention relates to field emission displays, and, more particularly, to a method and apparatus for reducing power consumption in field emission displays.
FIG. 1 is a simplified side cross-sectional view of a portion of a display 10 including a faceplate 20 and a baseplate 21, in accordance with the prior art. FIG. 1 is not drawn to scale. The faceplate 20 includes a transparent viewing screen 22, a transparent conductive layer 24 and a cathodoluminescent layer 26. The transparent viewing screen 22 supports the layers 24 and 26, acts as a viewing surface and forms a hermetically sealed package between the viewing screen 22 and the baseplate 21. The viewing screen 22 may be formed from glass. The transparent conductive layer 24 may be formed from indium tin oxide. The cathodoluminescent layer 26 may be segmented into pixels yielding different colors to provide a color display 10. Materials useful as cathodoluminescent materials in the cathodoluminescent layer 26 - include Y2O3:Eu (red, phosphor P-56), Y3(Al, Ga)5O12:Tb (green, phosphor P-53) and Y2(SiO5):Ce (blue, phosphor P-47) available from Osram Sylvania of Towanda PA or from Nichia of Japan.
The baseplate 21 includes emitters 30 formed on a surface of a substrate 32. The substrate 32 is coated with a-dielectric layer 34 that is formed, in accordance with the prior art, by deposition of silicon dioxide via a conventional TEOS process. The dielectric layer 34 is formed to have a thickness that is approximately equal to or just less than a height of the emitters 30. This thickness may be on the order of 0.4 microns, although greater or lesser thicknesses may be employed. A conductive extraction grid 38 is formed on the dielectric layer 34. The extraction grid 38 may be, for example, a thin layer of polycrystalline silicon. An opening 40 is created in the extraction grid 38 having a radius that is also approximately the separation of the extraction grid 38 from the tip of the emitter 30. The radius of the opening 40 may be about 0.4 microns, although larger or smaller openings 40 may also be employed.
In operation, signals coupled to the emitter 30 allow electrons to flow to the emitter 30. Intense electrical fields between the emitter 30 and the extraction grid 38 then cause field emission of electrons from the emitter 30. A positive voltage, ranging up to as much as 5,000 volts or more but generally 2,500 volts or less, is applied to the faceplate 20 via the transparent conductive layer 24. The electrons emitted from the emitter 30 are accelerated to the faceplate 20 by this voltage and strike the cathodoluminescent layer 26. This causes light emission in selected areas known as pixels, i.e., those areas adjacent to the emitters 30, and forms luminous images such as text, pictures and the like.
FIG. 2 is a simplified plan view showing rows 42 and columns 44 of the emitters 30 and the openings 40 of FIG. 1, according to the prior art. The columns 44 are divided into top columns 44a and bottom columns 44b, as may be seen in FIG. 2. Top 46a and bottom 46b column driving circuitry is coupled to the top 44a and bottom 44b columns, respectively. A row driving circuit 48 is coupled to odd rows 42a and even rows 42b. The rows 42 are formed from strips of the extraction grid 38 that are electrically isolated from each other. The columns 44a and 44b are formed from conductive strips that are electrically isolated from each other and that electrically interconnect groups of the emitters 30.
By biasing a selected one of the rows 42 to an appropriate voltage and also biasing a selected one of the columns 44 to a voltage that is about forty to eighty volts more negative than the voltage applied to the selected row 42, the emitter or emitters 30 located at an intersection of the selected row 42 and column 44 are addressed. The addressed emitter or emitters 30 then emit electrons that travel to the faceplate 20, as described above with respect to FIG. 1.
Conventional circuitry for driving emitters 30 in field emission displays 10 enables each column 44 once per row address interval and disables each column 44 once per row address interval. The columns 44 present a capacitive load C. Charging and discharging of the capacitance C consumes power in proportion to fCV2, where f represents the frequency of charging and discharging the column 44 and V represents the voltage to which the columns 44 are charged. Charging and discharging of the columns 44 in order to drive the emitters 30 forms a major component of the electrical power consumed by the display 10. As a result, reducing the frequency f, the capacitance C or the voltage V can significantly reduce the electrical power required to operate the display 10. Displays 10 requiring less electrical power are currently in demand.
There is therefore need for techniques and apparatus that reduce the amount of electrical power required in order to operate field emission displays.
In one aspect, the present invention includes a field emission display having a substrate and a plurality of emitters formed on the substrate. Each of the emitters is formed on one of a plurality of emitter conductors that is also a row or a column of the display. The display also includes a porous dielectric layer formed on the substrate and the columns. The porous dielectric layer has an 6opening formed about each of the emitters and has a thickness substantially equal to a height of the emitters above the substrate. The porous dielectric layer is preferably formed by oxidation of porous polycrystalline silicon. The display further includes an extraction grid formed substantially in a plane defined by respective tips of the plurality of emitters. The extraction grid has an opening surrounding each tip of a respective one of the emitters. The display additionally includes a cathodoluminescent-coated faceplate having a planar surface formed parallel to and near the plane of tips of the plurality of emitters.
The porous dielectric results in the emitter conductors having reduced capacitance C compared to prior art dielectric layers. Charging and discharging of the emitter conductors in order to drive the emitters forms a major component of the electrical power consumed by the display. By reducing the capacitance of the emitter conductors, the display is able to form luminous images, such as text and the like, while dissipating reduced electrical power.
In another aspect of the present invention, tips of the emitters are formed from a material having a work function less than four electron volts. The voltage needed in order to drive the emitters, and hence the voltage used to charge and discharge the columns, is proportional to a turn-on voltage for the emitters. Emitters having reduced turn-on voltage draw less electrical power. As a result, baseplates with emitters having low work function tips are able to form luminous images while dissipating reduced electrical power compared to conventional displays.